“Google Cardboard is a virtual reality (VR) platform developed by Google. Named for its fold-out cardboard viewer into which a smartphone is inserted, the platform was intended as a low-cost system to encourage interest and development in VR applications. Users can either build their own viewer from simple, low-cost components using specifications published by Google, or purchase a pre-manufactured one. To use the platform, users run Cardboard-compatible mobile apps on their phone, place it into the back of the viewer, and view content through the lenses.” — from Wikipedia
Although Google has stopped selling their version of the Cardboard Viewer, there are still many varieties made with cardboard, plastic, and other materials that are available from 3rd parties or you can very easily make your own viewer.